Down by the river

As Rebecca Brown kayaked down the Nolichucky River in North Carolina one summer, she followed a path similar to many of her own study subjects. Seeds and other propagules often float downstream before settling along riverbanks. Rampant with change, these areas offer a nutrient-rich location for new plants, yet pose the danger of sweeping vegetation away in a flood. It is this high volatility that makes the area resource rich and perfect for invasive and native plants to put down their roots. In a study presented in January’s Ecology, researchers Rebecca Brown and Robert Peet found areas subject to frequent flooding also showed a higher number of invasive exotic plants than upland regions outside of floodplains.

Mild to low-intensity disturbances, such as a small flood from a rainstorm every year, or larger floods every few years, create space and make nutrients available, allowing new plants to grow. Scientists call this an immigration process. In contrast to extinction processes such as competition, extreme disturbances, and environmental stresses, the immigration process sets the stage for new life.

“Community composition is driven by immigration. Areas disturbed frequently, such as riversides and roadsides, are more receptive to propagules from native species and also prove to be just as hospitable for exotics,” said Brown.

Brown and Peet, along with a team of field assistants, traversed the countryside collecting data along rivers and uplands in the southern Appalachian forests of North Carolina to compare the relationship between exotic and native species richness. Combining information from the Carolina Vegetation Survey database and the United States Department of Agriculture Plants database, the duo studied riparian areas within 100-year flood zones and uplands outside of the floodplains. They also took into account differences in soil pH, geology, and other factors, recording both herbs and trees in almost 1200 plots.

The researchers found species diversity to be significantly higher for both native and exotic species in the riparian areas than in upland areas.

“Riparian areas have roughly 40 times greater mean exotic species per plot then upland areas,” said Brown.

Even when examining areas with comparable amounts of light availability and soil fertility the results remained fairly similar. As flood frequency decreased, the number of exotic species decreased. But species richness — the variety of species — for native and exotics also decreased. According to the researchers these results may be a combination of fewer seeds and less disturbances, leading to fewer opportunities for exotics to invade the upland sites.

Brown and Peet also looked at the results in terms of scale.

Previous research suggests that high species diversity should reduce invasibility because the more species compete in an area, the fewer resources are available to incoming species. Brown and Peet’s study validated this concept at the small scale. However it did not hold true when they compared diversity of exotic and native species in large scale areas. Brown explains why this is so:

“It is possible that we see this phenomenon only at small scales because plants compete at small scales, while at large scales, flooding, variations in seed supply, or variations in resource availability allow the immigration of all types of new species, native and exotic, into the community.”

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